Method and apparatus for freezing products

ABSTRACT

A method of freezing a product, which method comprises the steps of vaporizing a cryogenic liquid and warming the vapor thus formed with a product to be frozen, characterized by using the warmed vapor thus obtained to cool compressed gaseous refrigerant in a mechanical refrigeration system before expansion thereof.

FIELD OF THE INVENTION

[0001] This invention relates to a method and apparatus for freezingproducts and, more particularly but not exclusively, is concerned with amethod and apparatus for freezing foodstuffs.

BACKGROUND OF THE INVENTION

[0002] The use of liquid nitrogen to freeze foodstuffs has increaseddramatically over the past 30 years. The improvement in the quality ofthe frozen food is well known. However, whilst liquid nitrogen is nowused for freezing premium food products its cost prevents it being usedfor freezing those foodstuffs which do not command a premium price.These foodstuffs are typically frozen using mechanical refrigeration.

[0003] Over the years many attempts have been made to reduce thequantity of liquid nitrogen required to freeze a given foodstuff andgradually it has become economically viable to use liquid nitrogen tofreeze an increasing range of foodstuffs.

[0004] The present invention aims to continue this trend. In particular,in existing cryogenic tunnel freezers the gaseous cryogen is typicallyvented at about −40° C. Although the emerging gas is still cold it istoo warm to have any practical value in the context of food freezing ina tunnel freezer. Normally, the cold vapour is vented direct toatmosphere. However, some installations pipe the cold vapour to indirectheat exchange coils inside a cold store where it is used to help reducethe heat load on an independent mechanical refrigeration system.

[0005] Whilst this arrangement does make some use of the residual coldit is not a particularly efficient use.

SUMMARY OF THE INVENTION

[0006] According to the present invention there is provided a method offreezing a product, which method comprises the steps of vaporising acryogenic liquid and warming the vapour thus formed with a product to befrozen without passing the vapour through a work expander, and using thewarmed vapour thus obtained to cool compressed refrigerant in amechanical refrigeration system before expansion thereof.

[0007] The cooled compressed refrigerant will normally be in the gaseousphase although it could also be condensed vapour.

[0008] Whilst the cryogenic liquid will usually be liquid nitrogen, itcould also comprise another cryogenic liquid, for example liquid air.

[0009] If desired, the vapour may be brought into direct heat exchangewith said product to refrigerate the same. Alternatively, the vapour maybe brought into indirect heat exchange with said product to refrigeratethe same.

[0010] In one embodiment, said mechanical refrigeration system has arefrigerated space and said method includes the step of passing saidproduct through said refrigerated space after freezing at least thesurface thereof with cryogenic fluid.

[0011] In another embodiment, said mechanical refrigeration system has arefrigerated space and said method includes the step of passing saidproduct through said refrigerated space before freezing it withcryogenic fluid.

[0012] Preferably, said product is a foodstuff.

[0013] The present invention also provides an apparatus for freezing aproduct, which apparatus comprises a freezer adapted to be cooled by acryogenic fluid without a work expander, a mechanical refrigerationsystem having a compressor, a heat exchanger for, in use, coolingcompressed refrigerant, and means for expanding said compressedrefrigerant, characterised by means for conveying cryogenic fluid fromsaid freezer to said or another heat exchanger to, in use, coolcompressed refrigerant prior to expansion thereof.

[0014] Advantageously, said mechanical refrigerator comprises arefrigerated space.

[0015] In one embodiment, said refrigerated space is disposed downstreamof said freezer.

[0016] In another embodiment, said refrigerated space is disposedupstream of said freezer.

[0017] In a further embodiment there are two refrigerated spaces (whichmay be associated with a single mechanical refrigerator or separate anddistinct mechanical refrigerators) one of which is disposed upstream ofsaid freezer and the other of which is disposed downstream thereof.

[0018] For a better understanding of the present invention referencewill now be made, by way of example, to the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic side elevation, partly in cross-section, ofone embodiment of an apparatus according to the present invention; and

[0020]FIG. 2 is a schematic side elevation, partly in cross-section, ofa second embodiment of an apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Referring to FIG. 1 there is shown a cryogenic storage vesselwhich is generally identified by reference numeral 110.

[0022] A pipe 111 is arranged to convey liquid nitrogen at minus 196° C.from the cryogenic storage vessel 110 and introduce it into a freezingtunnel 112 via a spray header 113 at a flow rate which is controlled byvalve 114 to maintain the temperature at a temperature sensor 128substantially constant.

[0023] The liquid nitrogen evaporates in the freezing tunnel 112 andrefrigerates food 115 being transported on a conveyor 116 through thefreezing tunnel 112. Heat transfer between the nitrogen vapour and thefood 115 is enhanced by a turbulence inducing fan 117 which is driven byan electric motor 118.

[0024] The nitrogen vapour leaves the freezing tunnel 112 and is thenblown by a fan 119 through duct 120 to a heat exchanger 121 where it isused to cool gaseous refrigerant in a mechanical refrigeration systemwhich is generally identified by reference numeral 122.

[0025] The mechanical refrigeration system 122 comprises a compressor123, a heat exchanger 124, water cooled heat exchanger 121, an expansionvalve 125 and a refrigeration coil 126 in a refrigerated space 127.

[0026] In use the pressure in the cryogenic storage vessel 110 is keptsomewhere between 1.5 and 3.5 bar g by a conventional evaporatorarrangement. In particular, when the pressure falls to 1.5 bar g a smallamount of liquid nitrogen is drawn off and evaporated in a local heatexchanger (not shown). Nitrogen expands by a factor of about 700 when itevaporates and the vapour is introduced into the top of the cryogenicstorage vessel 110. When the pressure reaches 3.5 bar g the flow ofnitrogen to the local heat exchanger is terminated and remainsterminated until the pressure in the cryogenic storage vessel 10 dropsto 1.5 bar g when the cycle is repeated.

[0027] It will be noted that the present invention does NOT utilise awork expander which is an essential feature of the applicants previouswork described in UK 9916487.3, UK 0003160.9 and overseas patentapplications corresponding thereto (all unpublished at the priority datehereof).

[0028] One of the problems with mechanical refrigeration is that inorder to achieve a significantly low temperature it is necessary toarrange several refrigerators in cascade with the refrigeration from thefirst refrigerator used to cool the compressed gas of the nextrefrigerator prior to expansion thereof.

[0029] Although a single refrigeration unit is relatively inexpensivethe capital cost increases significantly as the number of stagesincreases.

[0030] By using reject cryogenic vapour to cool the compressedrefrigerant in the mechanical refrigeraor prior to expansion significantrefrigeration can be achieved whilst saving at least one mechanicalrefrigeration stage.

[0031] An interesting further step is that instead of rejecting thecryogenic vapour at around the conventional temperature of −40° C. itmay be advantageous to reject the cryogenic vapour at a significantlylower temperature, for example −60° C. or colder, or even −70° C. orcolder.

[0032] As the temperature decreases the cryogenic freezing becomes lessefficient but the temperature of the mechanical refrigeration systemfalls and the speed of cooling in the mechanical refrigerator increases.Clearly there is a balance which can be optimised for any given set ofrequirements.

[0033] Turing now to FIG. 2, there is shown an apparatus which isgenerally similar to the apparatus shown in FIG. 1 and parts havingsimilar functions to parts shown in FIG. 1 have been identified by thesame reference numerals in the ‘200’ series.

[0034] The main differences are that the spray bar 113 has been replacedby an indirect heat exchanger 213 with the result that the fan 119becomes redundant. In addition the refrigerated space 227 is nowdisposed immediately downstream of the freezing tunnel 212 and theconveyor 216 carries the food sequentially through the freezing tunnel212 where it can obtain a crust freeze before passing through therefrigerated space 227 where it can equilibrate. The refrigerated space227 is provided with a turbulence inducing fan 229 to enhance heattransfer to the food 215 as it passes through the refrigerated space227. The expanded refrigerant entering the refrigeration coil 226 willtypically be at a temperature of from −40° C. to −80° C.

[0035] Various modifications to the arrangements described areavailable. For example, the refrigerated space 227 could be placedupstream of the freezing tunnel 212. Alternatively, a refrigerated spacecould be placed both upstream and downstream of the freezing tunnel 212.Each of such refrigerated spaces could be serviced by the samemechanical refrigerator or by separate and distinct mechanicalrefrigerators.

[0036] In order to maintain high standards of hygiene many tunnelfreezers are stopped and steam cleaned at frequent intervals, forexample every 24 hours for a single product freezer, or every 6 or 7hours when freezing small runs of gourmet products. Before the freezingtunnel can be reused it must be cooled down. This is conventionallyeffected by introducing liquid nitrogen into the freezing tunnel untilthe desired temperature is reached. It will be appreciated that whilstthe use of liquid nitrogen for initial cooldown is very quick it is alsovery expensive. Significant cost savings can be made by using externalelectrical power to mechanically cool the refrigerated spaces anddrawing the cold air therefrom through the freezing tunnel to achievepart of the initial cooldown.

[0037] As indicated previously, liquid air may be used as the cryogenicliquid although liquid nitrogen is preferred.

1. A method of freezing a product, which method comprises the steps of:(a) vaporising a cryogenic liquid and warming the vapour thus formedwith a product to be frozen without passing the vapour through a workexpander; and (b) using the warmed vapour thus obtained to coolcompressed refrigerant in a mechanical refrigeration system beforeexpansion thereof.
 2. A method according to claim 1, wherein saidcryogenic liquid is liquid nitrogen.
 3. A method according to claim 1,wherein the vapour is brought into direct heat exchange with saidproduct to refrigerate the same.
 4. A method according to claim 1,wherein the vapour is brought into indirect heat exchange with saidproduct to refrigerate the same.
 5. A method according to claim 1,wherein, said mechanical refrigeration system has a refrigerated spaceand said method includes the step of passing said product through saidrefrigerated space after freezing at least the surface thereof withcryogenic fluid.
 6. A method according to claim 1, wherein saidmechanical refrigeration system has a refrigerated space and said methodincludes the step of passing said product through said refrigeratedspace before freezing it with cryogenic fluid.
 7. A method as claimed inclaim 1, wherein said product is a foodstuff.
 8. An apparatus forfreezing a product, which apparatus comprises: (a) a freezer adapted tobe cooled by a cryogenic fluid without a work expander; (b) a mechanicalrefrigeration system having: (i) a compressor; and (ii) means forexpanding said compressed refrigerant, wherein said apparatus furthercomprises means for conveying cryogenic fluid from said freezer to aheat exchanger to, in use, cool compressed refrigerant prior toexpansion thereof.
 9. An apparatus as claimed in claim 8, wherein saidmechanical refrigerator comprises a refrigerated space.
 10. An apparatusas claimed in claim 9, wherein said refrigerated space is disposeddownstream of said freezer.
 11. An apparatus as claimed in claim 9,wherein said refrigerated space is disposed upstream of said freezer.